The present invention relates to the prevention of uncontrolled fire and/or explosion in waste gas disposal devices designed for the purging and/or controlled incineration of combustible waste gases, such as by-products of a reaction process.
Reference is made to my U.S. Pat. No. 4,661,056, issued Apr. 28, 1987, for its disclosure of an apparatus which is designed for the controlled incineration of waste gases and which functions by introducing the combustible waste gases under low pressure to an air conduit, through a controlled combustion conduit in which the waste gases are mixed with swirling air, ignited and then drawn into the air conduit and conveyed through a scrubber at the exit end of the air conduit for release as non-combustible reaction products.
Controlled combustion devices and purging devices for combustible waste gases must be capable of operation under low pressures since such waste gases commonly are by-products of reactions which take place at or slightly above atmospheric pressure and which require the laminar flow of combustible reaction gases or by-product gases through the reactor. Any attempt to increase the pressure of the waste gases as they enter the controlled combustion conduit can lead to back-pressure problems within the reactor. However, the exposure of the combustible waste gases at relatively low pressures, i.e., atmospheric or only slightly higher, to the ignition means in a combustion chamber, in the case of controlled combustion devices, or to an unintentional spark or other accidental ignition source within the transport conduit or scrubber, in the case of non-incineration purging systems, creates the danger that the controlled fire within the combustion conduit or accidental fire within the conduit might flash back upstream through the reactor exhaust pipe into the reactor or other processing equipment creating disruptive and possibly dangerous conditions. The flame propagation rate of hydrogen, for example, is about 8.25 feet per second, which permits the flame to travel upstream against the low pressure flow of a waste gas containing hydrogen.
The apparatus of my aforementioned Patent is an incineration apparatus which assists the mixing of air with the waste gas by creating a swirling action and vacuum within the air conduit, at the downstream end of the waste gas conduit, beyond a combustion chamber. However, such operation does not protect against high speed flame propagation or flash back.
It is known to use various commercially-available flame arresting devices and barriers within a combustion conduit in an effort to prevent flame from flashing back to the combustible gas inlet. Reference is made to U.S. Pat. Nos. 3,711,259 (porous, metal-coated plastic foam barrier); 3,748,111 (flame-arresting screen); 4,152,399 (fire screen); 4,444,109 (flame arresting membrane barrier); 4,555,389 (porous filler barrier and inert gases), and 4,613,303 (valved air screen). The devices of each of these patents are subject to failure in the event of malfunctions, and have no backup means to render them fail-safe. They rely upon the ability of a flame-arresting barrier to prevent flame from passing therethrough whereas such barriers are not completely reliable for this purpose against the high speed of flame propagation or flash back, particularly in the event of a malfunction, such as in the air supply means. Unless there is a sufficient rate of flow past the flame arrestor to maintain the flame at or downstream of the ignition source, the flame can burn in the upstream direction, permeate the flame arrestor and ignite a combustible mixture of the waste gas and air upstream of the flame arrestor, i.e., can flash back to the waste gas inlet. In such event the reaction apparatus must be shut down or diverted in order to stop feeding the combustible gasses to the combustor.